Method for recovering terephthalic acid crystals from slurry

Abstract: A method for recovering crystals from a crystal-containing slurry, which comprises supplying the slurry to a rotary filter having a rotating cylindrical filter medium, filtering the supplied slurry in a filtering region to retain the crystals on the filter medium and washing the filter cake formed on the rotating filter medium repeatedly in a plurality of washing regions by spraying a washing liquid onto the cake in each region by supplying any one of washing regions on the aft side in the rotating direction of the filter medium with the spent washing liquid of the washing region neighboring said one washing region on the fore side in the rotating direction.

Description: The present invention relates to a method for recovering terephthalic acid crystals from a slurry containing such crystals. Terephthalic acid used as a starting material of polyethelene terephthalate and the like is produced by oxidizing paraxylene with a molecular oxygen-containing gas in a reaction medium based on acetic acid, wherein the resulting terephthalic acid will deposit in the reaction liqour in a crystalline form. Since the so-formed slurry of the crystalline terephthalic acid contains also the acetic acid solvent, the unreacted paraxylene, by-products and the catalyst components, it is necessary to separate the terephthalic acid crystals, from the slurry in order then to recover the terephthalic acid. Fig. 3 shows a schematic sequence diagram for a conventional method for recovering terephthalic acid crystals from such a slurry. In Fig. 3, the numeral 101 denotes an oxidizing reactor, to which a raw material mixture 102 composed of paraxylene as the starting material, acetic acid as the reaction medium and the catalyst is supplied, while supplying thereto at the same time a molecular oxygen-containing gas 103 to cause oxidation of the paraxylene into terephthalic acid. In this conventional technique for recovering terephthalic acid crystals, the crystal-containing slurry 104 formed in the oxidizing reactor 101 is transferred to a first stage solid/liquid separator 105 to effect a primary solid/liquid separation to separate the crystals from the liquid phase. The liquid phase 106 composed mainly of acetic acid separated here is returned to the oxidizing reactor 101, while the crude crystals 107 are forwarded to a re-disperser 108, where the crystals are suspended again in a medium of acetic acid 109 fed thereto in order to dissolve out the contaminants included in the crude crystal mass. The resulting reformed slurry 110 is then subjected to a secondary liquid/solid separation to separate the crystals from the liquid phase in a second stage solid/liquid separator 111. The separated liquid phase 112 composed mainly of acetic acid is transferred to the oxidizing reactor 101, while the purified crystals 113 are forwarded to a drier 114. The dried crystals 115 are stored in a silo 116. The recovered crystals 117 are then conveyed to a disperser 118, whereto water 119 is also supplied and is mixed with the crystals 117 to form an aqueous slurry 120 which is guided then to a refinery system to refine the crystals. The conventional technique for recovering crystals from a slurry thus requires a large number of process steps with a correspondingly large number of devices and instruments, resulting in higher production costs for the final product. DE-A-1 278 402 discloses a rotary drum filter for separating and washing solid particles with a minimum of water. The rotary drum is hollow and has peripheral compartments connectable to the interior of the drum. Water filtered through a filter cake on the perforated periphery of the drum enters into these compartments and is collected and drained therefrom separately. The water is recycled. The water is supplied to the exterior of the drum from a succession of spray heads arranged around the exterior of the drum. The waste liquid originating from one spray head is collected and supplied to the spray head located immediately upstream of the first spray head with regard to the direction of rotation of the drum. BHS-Fest Druck-Vakuum Filter, 9 June 1961, Huttenwerk Sonthofen, discloses a rotary drum filter generally similar to that disclosed in DE-A-1 278 402. EP-A-0 406 424 discloses a method and apparatus for recovering crystals from a crystal-containing slurry using a rotary filter having a rotating cylindrical filter medium. Crystal-containing slurry is filtered in a filtering region of the rotating filter medium so as to separate the crystals as a filter cake on the filter medium. This filter cake is then washed in a washing region by supplying washing liquid to the exterior of the filter medium. Upon passing once through the filter cake the spent washing liquid is collected and stored in a waste tank prior to being disposed of. Where the crystals are crystals of terephthalic acid the spent washing liquid will contain acetic acid of low concentration. According to the present invention there is provided a method of recovering terephthalic acid crystals from a crystal-containing slurry using a rotary filter having a rotating cylindrical filter medium, the method comprising: supplying the crystal-containing slurry from a slurry preparation step to the outside of the rotating cylindrical filter medium whilst pressurizing with inert gas the slurry side of the rotary filter, the rotary filter having a plurality of washing regions including at least a first washing region and a last washing region, the last washing region being downstream of the first washing region in the direction of rotation of the filter medium; filtering the supplied slurry in a filtering region of the rotary filter so as to form a filtrate and to separate the crystals as a filter cake on the filter medium; washing the filter cake on the rotating filter medium in the plurality of washing regions by supplying washing liquid to the washing regions and collecting the spent washing liquid from each of the washing regions, the washing liquid supplied to said last washing region being fresh washing liquid and the washing liquid supplied to the first washing region being spent washing liquid collected from the washing region immediately downstream of the first washing region in the direction of rotation of the filter medium; supplying to the slurry preparation step the spent washing liquid collected from the first washing region, together with the filtrate from the filtering region; and collecting the washed filter cake from the filter medium. The rotating cylindrical filter medium may, for example, have three washing regions, said first washing region, said last washing region and an intermediate washing region provided therebetween, the washing liquid supplied to the intermediate washing region being the spent washing liquid collected from said last washing region and the washing liquid supplied to said first washing region being the spent washing liquid collected from the intermediate washing region. The present invention will be further described hereinafter with reference to the following description of an exemplary embodiment and the accompanying drawings, in which:- Fig. 1 is a schematic flow diagram of the method for recovering crystals from a slurry according to the present invention; Fig. 2 is a schematic sequence diagram for the method for recovering crystals from a slurry according to the present invention; Fig. 3 is a schematic sequence diagram for a typical conventional method for recovering crystals from a slurry. For the rotary filter to be employed in the method according to the present invention a rotary pressure filter operated by pressurizing the outside of its rotating filter medium for establishing the filtering pressure drop is used. A rotary pressure/sucking filter may also be used. In a rotary vacuum filter operated by maintaining a reduced pressure inside the rotating filter medium, a part of the solvent of the slurry becomes vaporized due to the lowering of the pressure, whereby the slurry temperature is lowered by being deprived of the latent heat of vaporization, so that the solubility of the solutes existing at their saturation points in the solvent of the slurry is decreased to force deposition thereof in a form of fine particles which fill up the liquid flow paths between the solid particles in the filter cake and cause stuffing of the filter medium. In contrast thereto, a rotary pressure filter and a rotary pressure/sucking filter undergo less tendency to stuffing of the filter medium than that of rotary vaccum filter, since occurrence of a slurry temperature lowering due to the solvent vaporization is avoided in them. According to the present invention, the filter cake formed on the filter medium is washed in a series of steps on the rotating filter medium in a plurality of washing regions subsequent to the filtering region of the rotary filter to a considerable degree, so that the number of solid/liquid separators and their accessories required for recovering the crystals are reduced as compared with those of the conventional techniques. The primary slurry containing the crude terephthalic acid crystals from an oxidizing reactor is filtered by a rotary filter on a rotating filter medium and the resulting filter cake is washed several times on the rotating filter medium with a washing liquid, whereupon the washed filter cake is re-suspended to form a secondary slurry which is sent directly to a disperser for forming the starting slurry to be supplied to the crystal refinery system. In this manner, a series of a primary solid/liquid separator, a re-disperser, a drier and a silo with their accessories in the first process step of the conventional technique are dispensed with. The contaminant components included in the filter cake can be recovered and reclaimed effectively by washing the filter cake on the rotating filter medium in a plurality of washing regions by supplying a washing liquid to and separating the spent washing liquid from each of the washing regions in such a manner that a fresh washing liquid is supplied to the washing region of the forefront in the moving direction of the rotating filter medium, while supplying any one of washing regions on the aft side of said forefront, seen in the moving direction of the rotating filter medium, with the spent washing liquid separated in the washing region neighboring said one washing region on the fore side, seen in the moving direction of the filter medium, and recycling the spent washing liquid from the washing region rearmost in the moving direction of the filter medium to the slurry preparation step together with the filtrate of the filtering region. In recovering terephthalic acid crystals by the method according to the present invention, the spent washing liquid from the final washing region, namely, the washing region rearmost in the rotating direction of the filter medium, has a considerably high acetic acid concentration due to the incremental accumulation thereof by the repeated use of the spent washing liquid from the preceding regions as the washing liquid successively. This spent washing liquid from the washing region rearmost in the rotating direction of the filter medium is recycled to the oxidizing reactor to reuse it as the reaction medium for the oxidation of paraxylene. In this manner, by the method for recovering crystals from a slurry according to the present invention, the procedures of repeated washing of the filter cake and solid/liquid separation of the slurry are realized in one and the same rotary filter, so that the number of solid/liquid separators to be incorporated and the accessories for them can be reduced, whereby the construction of the entire recovery system can be simplified and an econimization of energy and an improvement of the operation of the system are attainable. PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 shows a schematic flow diagram for recovering terephthalic acid crystals from a crystal-containing slurry according to a preferred embodiment of the present invention. In Fig. 1, the numeral 1 denotes an oxidizing reactor which is constructed so as to realize an oxidizing reaction of the raw material by supplying thereto the starting raw material mixture via a supply line L1 and a molecular oxygen-containing gas via a gas supply line L2. An exhaustion line L3 for discharging out the resulting slurry of the oxidizing reaction communicates from the bottom of the oxidizing reactor 1 to a flash drum 2. A slurry supply line L4 including a slurry pump 3 communicates from the bottom of the flash drum 2 to a rotary pressure filter 4 at a slurry inlet 5. The rotary pressure filter 4 having the slurry inlet 5 at its bottom has a casing 7 provided with a filter cake discharge gate 6 arranged downwards. A horizontal cylindrical filter medium 9 is disposed within the casing 7 of the rotary pressure filter 4 with its lower portion being immersed in a sump 8 of the slurry supplied thereto so as to constitute a filtering region 9a and is rotatingly driven clockwise in Fig. 1 at a constant speed by a not-shown motor under intermediation by a reduction gear. A plurality of spray nozzles 11a, 11b, 11c constituting a spraying unit for spraying a washing liquid onto the filter cake on the filter medium are disposed outside the rotating filter medium 9 each correspondingly, but in the reverse sense, in each of a plurality of washing regions 9b, 9c, 9d provided subsequently to the filtering region 9a above the slurry sump 8 and allotted successively in a row in the rotating direction of the filter medium. As a spent washing liquid receiving means for receiving the spent washing liquid sprayed onto the filter cake outside the filter medium and passed therethrough to the inside thereof, spent washing liquid receivers 12a, 12b are disposed inside the cylindrical filter medium in a paired relation with the spray nozzles 11a, 11b except the spray nozzle 11c of the washing region rearmost in the rotating direction. A gas blow nozzle 13 is located inside the filter medium 9 in a filter cake removal region 9e at a portion adjacent the cake discharge gate 6 and serves for blowing N 2 gas from inside to the filter cake layer to remove it off the filter medium 9. An exhaustion line L5 for exhausting the spent washing liquid for the rearmost spray nozzle in the rotating direction together with the filtrate collected in the bottom portion of the rotating filter medium communicates therefrom to a filtrate reservoir 14 which is connected with the oxidizing reactor 1 by a line L6 having a pump 15. A line L7 is connected with the nozzle 11a in order to supply pure water as the fresh washing liquid. The receiver 12a is connected with a first spent washing liquid storage tank 16 via a line L8 and the bottom of the storage tank 16 is connected with the spray nozzle 11b via a line L9 having a pump 17. The receiver 12b is connected with a second spent washing liquid storage tank 18 via a line L10 and the bottom of the storage tank 18 is connected with the spray nozzle 11c via a line L11 having a pump 19. The casing 7 is connected at its upper portion with a line L12 for pressurizing the outside of the rotating filter medium and its cake discharge gate 6 is connected with a disperser 21 via a line L13. The disperser 21 is connected with a line L14 for supplying pure water thereto and at its bottom with a line L15 having an interposed pump 22 and communicating to a refinery system. The flash drum 2 is connected with a condenser 23 by lines L16 and L17 for cooling the vapor from the flash drum 2 to condense it and to return the condensate thereto. The condenser 23 is connected with lines L18, L19 and L20 for supplying the cooling water thereto, for discharging the spent cooling water therefrom and for exhausting the trapped non-condensing gas out of it, respectively. The recovery of the crystals from the slurry using the apparatus explained above is carried out as follows: The oxidizing rector 1 is supplied with a raw material mixture composed of paraxylene as the starting material, acetic acid as the solvent and a catalyst via the line L1 and also with a molecular oxygen-containing gas, such as the air, via the line 2 and in which paraxylene is oxidized by the molecular oxygen into terephthalic acid. The resulting reaction product mixture in the form of a slurry is sent from the oxidizing rector 1 via the line L3 to the flash drum 2 where it is exposed to a reduced pressure and is flashed to cause a temperature decrease. The vapor phase is cooled by the condenser 23 to condense the condensible components into a condensate which is returned to the flash drum 2 via the line L17 and the non-condensible gas is exhausted out via the line L20. The slurry left in the flash drum 2 is guided via the line L4 to the slurry inlet 5 of the rotary pressure filter 4 by the pump 3. By rotating the horizontal cylindrical filter medium 9 clockwise (seen in Fig. 1) within the casing 7 of the rotary pressure filter 4 while pressurizing the outside of the cylindrical filter medium 4 by pressing an inert gas, such as N 2 , into the casing 7, the slurry contacting with the cylindrical rotating filter medium 9 in its bottom portion constituting the filtering region 9a immersed in the slurry is subjected to filtration by the pressure gradient across the filter medium to retain the crystals on the filter medium 9 as the filter cake while successively moving upwards in accordance with the rotation of the filter medium 9. The filter cake on the filter medium 9 passes then through the washing regions 9b, 9c and 9d in which it is washed with a washing liquid to remove off the contaminant components included therein. Here, pure water is supplied as the washing liquid via the line L7 to the spray nozzle 11a in the washing region 9d of the forefront in the rotating direction of the filter medium 9 and is sprayed to wash the filter cake. The spent washing water passed through the filter cake and the filter medium and collected in the receiver 12a is guided through the line L8 to the first spent washing liquid storage tank 16, from which it is supplied to the spray nozzle 11b of the washing region 9c neighboring the above-mentioned forefront washing region 9d in the aft side in the rotating direction of the filter medium by the pump 17 and is sprayed to wash the filter cake in the washing region 9c. The spent washing water collected in the receiver 12b of the washing region 9c is guided through the line L10 to the second spent washing liquid storage tank 18, from which it is supplied to the spray nozzle llc of the washing region 9b rearmost in the rotating direction of the filter medium by the pump 19 and is sprayed to wash the filter cake. By supplying, in this manner, any one of the washing regions on the aft side of the washing region of the forefront, seen in the moving direction of the rotating filter medium, with the spent washing water separated in the washing region neighboring the said one washing region on the fore side, seen in the moving direction of the filter medium, the concentration of the contaminant components, such as acetic acid and unreacted starting material, in the spent washing water becomes increased successively in the washing regions towards aft the rotating direction of the filter medium. The spent washing water of the washing region 9b rearmost in the rotating direction, which has the highest concentration of such contaminant components, is gathered with the filtrate from the filtering region 9a in the bottom portion of the horizontal cylindrical filter medium, from which it is recycled, namely, together with the filtrate, to the oxidizing reactor 1 via the line L6 by the pump 15 to reuse them in the oxidation reaction. In this manner, the filter cake of crystals retained on the rotating filter medium in a layer is cleaned to a considerable degree by a repeated washing (three times in this embodiment) in the washing regions 9b - 9d under separation of each spent washing water from the crystals with final washing by pure water, so that the contaminant components, such as acetic acid etc., occluded in between the crystals can efficiently be removed. The washed filter cake left on the rotating filter medium in a layer is blown off from the filter medium 9 in the filter cake removing region 9e by blowing an inert gas, such as N 2 gas, from inside onto the filter cake layer using a gas blowing nozzle 13 and the so-removed filter cake is discharged from the cake discharge gate 6 into the disperser 21, where the filter cake is re-suspended in pure water supplied thereto via the line L14 to form a secondary slurry which is supplied to the refinery system via the line L15 by the slurry pump 22. In Fig. 2, the procedures of the method according to the present invention described above are explained schematically in a flow sequence diagram. As shown, the slurry containing crystals of terephthalic acid formed in the oxidizing reactor 1 is guided to the rotary pressure filter 4 via the line L3,L4. In the rotary pressure filter 4, a plurality of solid/liquid separation steps of filtration and washing purification are performed in the filtering region 9a and the washing regions 9b, 9c and 9d, wherein the washing liquid supplied via the line L7 is used for washing the filter cake in the washing region 9d in the forefront in the rotating direction of the filter medium and the spent washing liquid thereof is used for washing the filter cake in the washing region 9c neighboring the said forefront washing region 9d in the aft side in the rotating direction of the filter medium by supplying it thereto as the washing liquid therefor via the line L8,L9 and, further, the spent washing liquid of this washing region 9c is used for washing the filter cake in the washing region 9b rearmost in the rotating direction of the filter medium by supplying it thereto as the- washing liquid therefor via the line L10,L11. The went washing liquid separated in the washing region 9b rearmost in the rotating direction of the filter medium is returned together with the filtrate from the filtering region 9a to the oxidizing reactor 1 via the line L5,L6. The filter cake of the crystals washed finally by pure water in the washing region 9d in the forefront in the rotating direction is collected and sent to the disperser 21 via the line L13. As described above, the method according to the present invention can afford to recover terephthalic acid crystals from a crystal-containing slurry using a simplified apparatus by a simple operation efficiently with the simultaneous attainment of an effective reclamation of the contaminant component included in between the crystals in the filter cake, i.e. acetic acid., by recovering it at higher concentrations, since the procedures of washing of the filter cake and separation of the spent washing liquid are repeated in a plurality of washing regions settled within the rotary filter by using the spent washing water separated in any one of the washing region as the washing liquid for the washing region neighboring the said one washing region on the aft side in the moving direction of the filter medium.

Claims: A method of recovering terephthalic acid crystals from a crystal-containing slurry using a rotary filter (4) having a rotating cylindrical filter medium (9), the method comprising: supplying the crystal-containing slurry from a slurry preparation step to the outside of the rotating cylindrical filter medium (9) whilst pressurizing with inert gas the slurry side of the rotary filter (4), the rotary filter (4) having a plurality of washing regions (9b, 9c, 9d) including at least a first washing region (9b) and a last washing region (9d), the last washing region (9d) being downstream of the first washing region (9b) in the direction of rotation of the filter medium (9); filtering the supplied slurry in a filtering region (9a) of the rotary filter (4) so as to form a filtrate and to separate the crystals as a filter cake on the filter medium (9); washing the filter cake on the rotating filter medium (9) in the plurality of washing regions (9b, 9c, 9d) by supplying washing liquid to the washing regions and collecting the spent washing liquid from each of the washing regions,the washing liquid supplied to said last washing region (9d) being fresh washing liquid and the washing liquid supplied to the first washing region (9b) being spent washing liquid collected from the washing region (9c) immediately downstream of the first washing region (9b) in the direction of rotation of the filter medium (9); supplying to the slurry preparation step the spent washing liquid collected from the first washing region (9b), together with the filtrate from the filtering region (9a); and collecting the washed filter cake from the filter medium (9). A method as claimed in claim 1, wherein the rotary filter (4) has three washing regions (9b, 9c, 9d) comprising said first (9b) and last (9d) washing regions and an intermediate washing region (9c) provided therebetween, the washing liquid supplied to the intermediate washing region (9c) being spent washing liquid collected from said last washing region (9d) and the washing liquid supplied to said first washing region (9b) being spent washing liquid collected from the intermediate washing region (9c).

Inventors: OKAMOTO ETSURO, SAKATA TOSHIYUKI, SUZUKI HIROSHI, YAMAMOTO RYOICHI, OKAMOTO, ETSURO, SAKATA, TOSHIYUKI, SUZUKI, HIROSHI, YAMAMOTO, RYOICHI, OKAMOTO, ETSURO, C/O MITSUI PETROCHEM. IND., LTD, SAKATA, TOSHIYUKI, C/O MITSUI PETROCHEM. IND., LTD, SUZUKI, HIROSHI, C/O MITSUI PETROCHEM. IND., LTD, YAMAMOTO, RYOICHI, C/O MITSUI PETROCHEM. IND., LTD
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1994
Main classification: B01D33
IPC codes: B01D24, B01D33, B01D35, B01D29